An acoustic signal is altered by various processes during long-range propagation. Birds may use degraded sound features to assess the distance of the emitter. To know which temporal sound features are susceptible to be used by birds for ranging, we performed a study using natural and synthetic acoustic signals which had different temporal sound degraded characteristics, i.e amplitude and frequency modulations and sound-silence alternations.
Our data show that the degradation of some temporal features may give ranging information, especially in a forest environment. Indeed, in an open field, the assessment of an emitter's distance using sound temporal features may be rather difficult since there is no modification of sound/silence alternation, frequency-modulated notes are preserved, and fluctuations of amplitude are at random, except for high-pitched sounds. In contrast, forest birds may rely on different parameters for ranging, since in this environment duration of notes, degradation of frequency modulation as well as degradation of amplitude modulation of high-pitched notes are linked with propagation distance.